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  1. #1

    Expander Mandrels....

    So this has been stuck in my pee brain for awhile now.

    Instead of controlling neck tension by using a Bushing die, why donít we use an expander mandrel on the last step as itís the true inside diameter that defines neck tension?

    My thought is this..... I have a whole drawer full of RCBS FL Dies in various calibers. Could I pull the expander balls out of these dies (decap in separate step), bump shoulders .002-.003 and then use a expander mandrel sized about .001 smaller then bullet diameter (with springback should generate about .002 neck tension) on the inside of the neck with imperial lube?

    I already have the K&M Mandrel, and can get one for each caliber diameter that I need. Iíve found the expander balls vary greatly in size even in the same caliber.

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  2. #2
    LRO Owner ~ Review Editor ~ Long Range Hunting Specialist Broz's Avatar
    Join Date
    Apr 2013
    At the foot of the Big Belt Mountains near Townsend Montana
    Some of the bushing dies come with expander mandrels, (and an option to remove it) that expand on the up stroke after sizing. Most people who don't use expander mandrels do so to not induce run out or over work necks.

    Believe it or not, I have close to $2500 invested in the dies used to load my ELR 375. It does not use an expander mandrel. I think this is one of those things where a guy had to determine what works best for you and go with it.


  3. #3
    Senior Member
    Join Date
    Dec 2016
    BC, Canada
    I accomplish that with my redding type s fl dies. I buy there carbide size button which free floats on the stem unlike the other buttons that are solid. That way it can self center to the neck and not induce any runout. The button is sized to give you .002 neck tension.

    I set up the die by measuring loaded rounds and I pick a bushing that is .002-.003 smaller than a loaded round. Since the button also set to give .002 neck tension it draws through the neck very easily and with its ability to center to the neck there is no runout induced. This method ensures that the inside of the necks are all the same and any variances in brass wall thickness is on the outside of the neck and not affecting neck tension. It also ensures that any dings to the neck that the bushing alone would just push inward are taken out.

    I realize that most don't run the button with there bushing dies but if your not neck turning and checking for any kind of deformation of your necks, then the variance will end up on the inside of the neck. What that could or would do, could be debated for ever but the fact is any variation or deformation will end up on the inside of the neck.

    The difference with this and a standard die is that a standard die sizes the neck to small in most cases and then the standard, solid mounted button drags through the neck with a lot of force and can induce runout. With the right size bushing combined with the free floating button, the button just tickles through the neck but ensures the inside of the necks are all uniform.

    I also lube my cases with Hornady one shot lube misted from all four sides on about a 30 degree angle this method also gets just a light bit of lube on the inside of the neck so the button has even less friction to do its work.

  4. #4
    I've talked to people that use K&M mandrels to control their neck tension instead of bushings and they claimed great success. Its been awhile and I don't exactly recall all the benefits they were claiming but I recall them saying the use of K&M mandrel, after sizing, gave them straighter necks and less runout.

    I have never used my K&M mandrel to control neck tension but I use mine on all new brass to make the necks round and true.

  5. #5
    I have had good luck loading on Forster fl sizing does and then using a mandrel from Brownells after. I use the carbide expander and I have a turning mandrel so I can get a little more neck tension. You can order one that honed by Forster so that you do not get as much runout. I get less runout this way rather than using Redding competition, but that works for me. Obviously broz has forgotten more about loading and shooting than I ever will which is awesome. I am a high school football coach so funds are not readily available, and I have tried to find an economical way. (I take expander ball out by the way)

    Redding comp dies made great ammo but I always got a lot of runout. I admit I probably did something wrong. I like the Forster competition seater so much better itís all I use and itís only like 30-50 dollars cheaper.

    Steel expander are really cheap from brownells as well and easier than k&m. Carbide is higher.

  6. #6
    I use standard FL dies and run cases through a Lee collet die in a couple cases. Loads very straight ammo with consistent neck tension
    Cheap and effective

  7. #7

    I think it is a search for an answer to a problem that does not exist. I can typically get .001 runout using Redding busing dies with the decapping assembly not tightened such that the bushing cannot move. I also seat bullets roughly half way and then rotate to finish using a Redding competition die. John Whidden claims to have better success using the method in question. Obviously, he makes it work for him but I have not had the same luck.

    Expander Ball Kits – Whidden Gunworks

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